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The protocol for discovering an XRDS document from a URL was formalized as the Yadis specification published by Yadis.org in March 2006. Yadis became the service discovery format for OpenID 1.1.

A common discovery service for both URLs and XRIs proved so useful that in November 2007 the XRI Resolution 2.0 specification formally added the URL-based method of XRDS discovery (Section 6). This format and discovery protocol subsequently became part of OpenID Authentication 2.0.

In early 2008, work on OAuth discovery by Eran Hammer-Lahav led to the development of XRDS Simple, a profile of XRDS that restricts it to the most basic elements and introduces some extensions to support OAuth discovery and other protocols that use specific HTTP methods. In late 2008, XRDS Simple has been cancelled and merged back into the main XRDS specification resulting in the upcoming XRD 1.0 format.

Following is an example of an XRDS document for the fictional XRIi-name=example. This document would typically be requested from a Web server via HTTP or HTTPS using the content type application/xrds+xml. Note that the outer container <XRDS> element serves as a container for one or more <XRD> (Extensible Resource Descriptor) elements. Most simple XRDS documents have only one XRD. Other services like XRI resolution may construct a sequence of XRDs within a single XRDS document to reflect a chain of metadata about linked resources.

XRDS documents can assert zero or more synonyms for a resource. In this context, a synonym is another identifier (a URI or XRI) that identifies the same target resource. For instance, the example XRDS document above asserts four synonyms:

The local synonym !4C72.6C81.D78F.90B2. This is a relative XRI synonym assigned by the provider of this XRDS document.

The equivalent URLhttp://example.com/example-user with a priority of 10 (1 is the highest priority).

The equivalent URLhttp://example.net/blog with a priority of 15 (a lower priority than the other equivalent URL above).

The canonical identifier xri://=!4C72.6C81.D78F.90B2. This is an absolute XRIi-number for the target resource—a persistent identifier that will never be reassigned (the functional equivalent of a Uniform Resource Name).

The other main purpose of XRDS documents is to assert the services associated with a resource, called service endpoints or SEPs. For instance, the example XRDS document above asserts four service endpoints for the represented resource:

In XRDS documents, a service is identified using a URI or XRI. Following are listings of well-known service types. See also XRDS Type, an open community effort begun in May 2008 to provide a catalog of XRDS service types.